DESCRIPTION: The risk of occupationally-related low-back disorders (LBD) in women is twice that for men. In general, LBDs are the leading cause of lost work days and the most costly occupational safety and health problem facing industry today. As the workforce approaches gender equity in numbers and occupational roles, the rate of LBDs will rise proportionally. Unfortunately, the influence of gender as a risk factor for musculoskeletal injury and LBD is poorly understood. Low-back pain develops when spinal load repetitively exceeds injury tolerance. Although there is no evidence suggesting spinal load is greater for women than men, there are studies indicating possible gender differences in spinal stability. Stability determines the maximum load the spine can withstand safely, i.e. injury tolerance. Spinal shape, muscle recruitment, muscle stiffness, and reaction mechanics contribute to spinal stability. There is limited research suggesting gender differences in these factors. Thus, men and women are exposed to similar spinal loads during manual materials handling (MMH) tasks, but women may have a lower tolerance to spinal load resulting in a higher risk of occupational LBD. Size and strength are obvious characteristics differentiating men and women from a MMH standpoint. This difference results in greater potential for fatigue in women compared to equivalently conditioned men. Fatigue has been shown to influence spinal shape, muscle recruitment patterns, muscle stiffness, and reaction mechanics. Hence, fatigue may be related to spinal stability and associated LBD risk. The potential for musculoskeletal instability and LBD injury in women is enhanced in fatiguing environments. To explain the increased risk of occupational LBD we propose three hypotheses. 1) A gender difference exists in spinal stability factors including dynamic spinal shape, reflex time and magnitude, muscle stiffness (elasticity), and muscle recruitment patterns (co-contraction). 2) Fatigue enhances the gender differences in spinal stability factors. 3) These musculoskeletal factors contribute to gender differences in the stability of the spine during lifting tasks. We propose to quantify these musculoskeletal factors as a function of gender and fatigue under controlled and MMH lifting conditions. Musculoskeletal stability and associated injury tolerance will be modeled from these data as a function of gender and fatigue. Results will demonstrate how workplace factors influence women differently than men and identify injury mechanisms explaining increased risk in women. The goal of this research is to permit greater inclusion of women in the workforce while controlling the risk of occupational LBD.